It's a puzzle to scientists, but a new study suggests that the main cause of deadly skin cancer -- sunlight -- might also help protect against the disease.

The key could lie in the amount of ultraviolet B (UVB) light the skin absorbs -- enough to stimulate a healthy, vitamin D-linked immune response in the skin but not so much that it boosts skin cancer risk.

"I do think that a little bit of sunlight is good for people, but I think that one of the problems that the American Cancer Society and dermatologists have is, how do you define what a little bit is?" said skin cancer researcher Marianne Berwick, chief of epidemiology at the University of New Mexico's Cancer Research and Treatment Center. "How do you tell people that it's OK to have a little bit of sunlight but not too much?"

In 2005, Berwick's team published a controversial study that found that melanoma patients with higher levels of daily sun exposure actually had better survival than patients who spent less time in the sun.

"I've been searching for an explanation for that ever since," she said.

Now, findings from a group led by immunologists at Stanford University may provide an answer. The study, led by professor of pathology Eugene Butcher, is expected to be published in the March issue of Nature Immunology.

In its study, the Stanford team worked with cells in the lab and discovered a biochemical chain of events that appears to link sunlight exposure to the skin's own immune defenses.

The researchers started from the notion that an inactive precursor of vitamin D, called vitamin D3, "is generated in the skin in response to sun exposure." That's been known for years. Specifically, a short-wavelength form of UV light, called UVB, is responsible for D3 generation.

D3 is inert and powerless, however. Through contact with various enzymes in the liver and kidneys, the body turns D3 into an active compound called 1,25(OH)2D3.

And that's where the immune-system connection kicks in, the Stanford authors said.

In their experiments, they found that the new compound "signaled (immune) T-cells," pushing them to migrate back to specific sites in the skin's epidermis. Once there, these powerful immune system agents stand on guard against infection and even cancer, the researchers said.

"So, the same wavelengths of sunlight that are most potent in inducing skin cancer -- UVB -- are also the wavelengths that produce this vitamin D precursor, D3," said Dr. Martin Weinstock, chairman of the skin cancer advisory group at the American Cancer Society. And it's D3 that starts the whole chain of events rolling.

Weinstock stressed that the Stanford study is far from conclusive, however, and should not be seen as an excuse to bake in the sun.

"We know that the sun is the major avoidable cause of skin cancer," he said. "This study is interesting and points to a productive area of research, both to confirm this in other settings and to flush out the implications of the finding. But does it really relate to skin cancers in real live people? We don't know."

"So, avoiding intense sun, protecting yourself when you're out in intense sun -- that's still our [cancer society] recommendation, and this is not going to change that," said Weinstock, who is also professor of dermatology and community health at Brown University.

Kathleen Egan, a professor of epidemiology at the H. Lee Moffitt Cancer Center and Research Institute in Tampa, Fla., agreed that the study findings are "tantalizing" but need further study.

Especially since the release of Berwick's melanoma study, "there's been an awful lot of questions about how -- or if -- vitamin D has a part to play in potentially offering some [cancer] protection under some circumstances," she said. "But it's very difficult to tease out, because the main human source of vitamin D is, in fact, sunlight exposure, which is also the most important risk factor for melanoma."

Nutritionists have known for decades that sunlight stimulates vitamin D production in the skin. In fact, this natural process is the body's major source of the nutrient. A proper amount of vitamin D is crucial to bone health, "and there's also a bunch of evidence that vitamin D may have a role in preventing colon cancer, although there's still some controversy about that," Weinstock said.

So, how much sunlight is enough to get the ideal amount of vitamin D?

Katharine Tallmadge, a Washington, D.C., dietitian and a spokeswoman for the American Dietetic Association, suggests that most people can probably get the U.S. Department of Agriculture's recommended 400 daily IUs of vitamin D by spending a half-hour to an hour outside per day.

Egan agreed. She said it's not difficult for people to soak up the sun's goodness without boosting their cancer risk. In response to even a moderate amount of sunlight, "the skin actually creates an amazing amount of vitamin D," Egan said. "It doesn't take much exposure to make enough of the vitamin D that's certainly needed to preserve bone health, for example."

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